In the lower worlds when you are fighting a boss and you manage to kill the boss minions yet are still killed by the boss when you returned to the instance only the boss remained. Now that I am in higher worlds I've noticed the nasty trend of the minions returning thus creating an instance you already got killed so the player just gets killed again.
This is maddening and feels very unfair.
Sometimes I think the sole purpose of developers is just to discourage and upset us.
Not sure which worlds you are referring to, but I have noticed (currently in Avalon) that some boss fights it seems random whether or not the minions return.